Analysis of First Pressure Vessel Surveillance Capsule from Kršsko Nuclear Power Plant

Results of the analysis of the first pressure vessel (PV) radiation embrittlement surveillance capsule from Krško nuclear power plant are presented. The capsule was removed and tested after 1.75 effective full power years of operation. Neutron flux and spectrum were calculated with DOT 3.5 and 4.2 transport codes at the locations of the specimens in the capsule and in the pressure vessel. Nickel, iron, copper, cobalt, Np-237, and U-238 neutron dosimeters were irradiated in surveillance capsules. Experimentally determined reaction rates were compared to values calculated from the neutron transport calculation,with source strength corresponding to the nominal reactor thermal power. Good agreement (within 15%) was found for threshold neutron detectors. The STAY'SL unfolding code was used to adjust the measured and calculated reaction rates and neutron fluences.

Shifts in the RTN temperature determined from Charpy V-notch curves for irradiated material relative to that of unirradiated materials were compared with shifts predicted by NRC Regulatory Guide 1.99, Rev. 1 and Rev. 2, at the neutron exposure of 7 to 9 E18 n∙cm−2 (ϕ(E > 1 MeV)) experienced by specimens taking into account the chemical composition of PV steel.